1. Field of the Invention
The present invention generally relates to polyester polyols that are liquid at room temperature and more particularly to polyester polyols based on 2-methyl1,5-pentanediol; 2-ethyl-1,4-butanediol; 2-methyl glutaric acid and/or 2-ethyl succinic acid.
2. Description of the Relevant Art
The preparation of polyester polyols and their use in the preparation of polyurethane plastics using the polyisocyanates addition polymerization process is well known and is described in numerous literature and patent publications. Examples are found in the Plastics Handbook, Volume 7, Polyurethanes, first edition, 1966, edited by A. Hoechtlen and R. Vieweg, and the second edition, 1983, edited by G. Oertel and Carl Hanes Verlag, Munich and Vienna.
Polyester polyols are commonly prepared from polycarboxylic acids and multivalent alcohols by the molten condensation or by the transesterification of dicarboxylic acids in the presence of acids or metal salts as catalysts. Preferably, the polycondensation is carried out under reduced pressure in order to distill off the water resulting from the condensation or in order to distill off the alcohol cleaved off from the reaction mixture. At the same time, the equilibrium reaction is moved in the direction of polyester formation. In this fashion polyester polyols are prepared having acid numbers larger than 5 and with relatively short reaction times.
Although polyester polyols, particularly those based on adipic acids and alkanediols, alkanediol mixtures or mixtures of alkanediols and higher valent alcohols, are used in large quantities for the preparation of polyurethane plastics for a number of areas of applications, the products possess disadvantages. One particular disadvantage is that the polyester polyols either completely or partially crystallize out under normal storage conditions, or they exhibit such a high viscosity that they cannot be processed directly at room temperature using the polyisocyanate addition polymerization process with conventional equipment. The polyester polyols must be routinely melted down in externally protected equipment and homogenized and only then are able to be reacted at high temperatures in costly temperature controlled devices. The preparation of the polyurethane finished articles is in turn made more expensive.
In order to avoid this disadvantage, following the specifications of EPA-No. 017060 for the preparation of polyurethanes elastomers and following the specification of EPA No. 056,122 (U.S. Pat. No. 4,362,825) in preparing flexible polyurethane flexible foams, liquid polyester polyols having hydroxyl numbers of from 40 to 80 are used in certain defined quantity ratios, which are obtained by the polycondensation of organic dicarboxylic acids with polyol mixtures comprising at least 4 divalent and trivalent alcohols based on 1,4-butanediol, 1,6-hexanediol, 1,5-pentanediol and/or diethylene glycol and an alkanetriol such as glycerine and/or trimethylolpropane. The polyurethane elastomers obtained in this fashion have a favorable cold tolerance and good hydrolysis resistance at a high level of mechanical strength, as well as exhibiting a very good compressive permanent set and the polyurethane flexible foams exhibit both improved hydrolysis resistance and excellent punching ability. Polyol mixtures of this kind are relatively expensive since the starting components are obtained partially by the reduction of the corresponding dicarboxylic acid mixtures, and are only available in limited quantities for the preparation of polyester polyols.
Polyester polyols, prepared by the polycondensation of mixtures comprising succinic, glutaric and adipic acids and multivalent alcohols, preferably aliphatic alcohols, are likewise not new. According to EPA No. 0044969, the products are suited for the preparation of thermoplastic polyurethane elastomers, which have a good mechanical property level, especially improved tensile strength at break and abrasion resistance. According to both Great Britain Patent Nos. 882,603 and 1,475,541, the products are used as starting materials for polyurethane foams.
German Patent No. 3,614,038.4 likewise discloses low viscous polyester polyols, liquid at room temperature, which contain in bonded form units having the structure --OCH.sub.2 --CH(CH.sub.3)--CH.sub.2 --CH.sub.2 O--. Plastics prepared from these polyester polyols using the polyisocyanates addition polymerization process, particularly non-cellular or cellular polyurethane elastomer or polyurethane-polyurea elastomers, have a high mechanical property level and good hydrolysis stability.
In addition, disclosed in DE-OS-No. 2609207 (U.S. Pat. No. 4,052,358) are pigmentable unsaturated polyester resin compositions having low shrinkage, which comprise as glycol components a mixture of from 15 to 70 mole percent of neopentyl glycol and from 10 to 30 mole percent of 2,2-bis (4-hydroxycyclohexyl)-propane and where the glycol components additionally can contain from 1 to 15 mole percent of a multivalent alcohol.
Polyurethane elastomers, which are prepared by the reaction of higher molecular polyhydroxyl compounds, for example polyester polyols or polyether polyols, organic polyisocyanates, preferably aromatic diisocyanates and lower molecular chain extending agents, find numerous applications. They are described in numerous patent and literature publications, such as in the above-mentioned Plastics Handbook, Volume 7, Polyurethanes, or in Plastics 68 (1978), pp. 819-825, as well as in the literature references cited therein.
Along with other mechanical properties such as mechanical strength, elongation, elasticity, and the like, polyurethane elastomers are also required to have particularly good hydrolysis resistance and cold stability. These properties are determined essentially by a suitable selection of the starting components forming the polyurethane elastomers. Polyurethane elastomers based on polyester polyols are relatively susceptible to hydrolysis. This hydrolysis stability expresses itself in a constant decline in mechanical property values following long term exposure to water or steam. The hydrolysis stability of elastomers can be improved by using certain polyester polyols, such as 1,6-hexanediol-polyadapates. Polyurethane elastomers from 1,6-hexanediol-polyadapates indeed demonstrates sufficient hydrolysis resistance, however, a disadvantage is the high crystallization tendency of the 1,6-hexanediol-polyadapates, which is expressed by poor cold resistance from the polyurethane elastomers; the polyadapates also have a high melt point, which generally causes processing problems.
By using diol mixtures, for purposes of interfering in the crystallization, in preparing polyester polyols the cold behavior of the polyurethane elastomers can be improved, however, the hydrolysis stability of the polyurethane elastomers is in turn impaired.
The objectives of the present discovery were to develop polyester polyols having a viscosity as low as possible that are liquid at room temperature and which can be processed problem free with conventional equipment using the polyisocyanate addition polymerization process into non-cellular or cellular plastics having good mechanical property levels.
The polyester polyols, when used in preparing polyurethane elastomers, should provide products having a high hydrolysis resistance and at the same time offer improved cold resistance over the state of the art.
This objective was surprisingly met by the complete or partial use of 2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol, 2-methyl glutaric acid and/or 2-ethylsuccinic acid as well as optionally other organic polycarboxylic acids and multivalent alcohols in the preparation of the polyester polyols.